Peripheral nerves consist of bundles of nerve fibers surrounded by three connective tissue sheets. Nerves can be sensory, motor, or mixed. Large diameter fibers conduct faster and have a lower threshold, while small fibers conduct slowly and are involved in pain, temperature, and autonomic functions. Mixed nerves contain different fiber types and exhibit compound action potentials with multiple peaks corresponding to different fiber groups. Classifying fibers based on diameter, conduction velocity, and function helps evaluate their roles and the effects of drugs.

1. Classification of nerve
fibers
Dr. Sai Sailesh Kumar G

2. Peripheral nerve
It consists of bundle of nerve fibers
Three connective tissue sheets
Endoneurium –covers single nerve fibre
Perineurium – covers a bundle of nerve fibers
Epineurium – entire nerve trunk is surrounded

3. Peripheral nerve
May be purely sensory or motor or mixed nerve
Both myelinated and unmyelinated
Both small and large diameter fibers

4. Large diameter fibers
Conduct impulses faster than small diameter fibers
Less threshold for excitation
Refractory period is shorter
Concerned with proprioception, touch, pressure, and somatomotor function

5. Small diameter fibers
Conduct impulses slowly
Concerned with pain, temperature and autonomic functions

6. Properties of mixed nerve
Peripheral nerves usually is mixed nerves
Consists of nerve fibers of different diameters
Apart from the properties of nerve fibers, the peripheral nerve also has two
important properties
Compound action potential
Maximal stimulus

7. Compound action potential
Place two electrodes on the surface of the mixed nerve
AP recorded with multiple peaks
Mixed nerve has many nerve fibers with different conduction velocities
Hence each peak for a particular group of nerve fiber
First peak belongs to fastest conducting fiber
Last peak for slowest conducting fibers

8. Compound action potential
Helps to determine the conduction velocity of different types of nerve fibers
To evaluate effect of certain drugs such as local anaesthetics

10. Maximal stimulus
Mixed nerve has different types of nerve fibers
To excite all of them
Maximal stimulation is needed
threshold stimulus may activate only few fibers

11. Classification of nerve fibres
Numerical classification
Sensory nerve fibers are classified numerically
Ia- Muscle spindle, annulospiral endings
Ib- Golgi tendon organ
II – muscle spindle- flower spray ending, touch, pressure
III-pin, temperature, some touch receptors
IV- pain

12. Classification of nerve fibres
Erlanger- Grasser classification
Based on diameter and conduction velocity

14. Classification of nerve fibres
Physio clinical classification
Functional

16. On the basis of myelination
Myelinated nerve fibers – enveloped by myelin sheath
Unmyelinated nerve fibers – do not have myelin sheath

17. Depending on the function
Sensory fibers – carry sensory impulses to the CNS
Motor fibers – carry motor impulses to muscles and glands

18. Depending on NT released
Cholinergic fibers –Release Ach
Adrenergic fibers- Release nor epinephrine
Dopaminergic fibers – releases dopamine

19. Common causes of nerve injury
Compression of nerve – ischemia
Transection of nerve
Crush injuries
Toxic drugs
Hyperpyrexia -condition where the body temperature goes above 106.7 degrees
Fahrenheit (41.5 degrees Celsius) due to changes in the hypothalamus

20. Types of nerve injuries
According to Sunderland, nerve injuries are graded into five types
Seddons classification is given in brackets

21. First degree nerve injury (neurapraxia)
Commonly seen
Secondary to ischemia caused by pressure on the nerve for a definite perod
Nerve losses its function temporarily due to ischemia
Recovery is rapid and complete
No evidence of nerve degeneration
Saturday night paralysis

22. Second degree nerve injury (Axonotmesis I)
Compression of nerve for prolonged periods
Damage to nerve at pressure point
Axons undergo degeneration from the pressure point to the distal end
Regeneration is possible
Functional recovery is fairly rapid and complete
Eg: Carpal tunnel syndrome

23. Second degree nerve injury (Axonotmesis I)
Carpal tunnel syndrome is a common condition that causes numbness,
tingling, and pain in the hand and forearm.
The condition occurs when one of the major nerves to the hand — the
median nerve — is squeezed or compressed as it travels through the wrist.

24. Third degree nerve injury (Axonotmesis II)
Along with axon
Endoneurial tube also disrupted
Recovery is slow, poor and incomplete

25. Fourth degree nerve injury (Axonotmesis III)
Fascicle is disrupted
Damage to perineurium
Regeneration is poor and incomplete

26. fifth degree nerve injury (Neurotmesis)
Along with axon all the three sheaths are damaged
Complete section of nerve trunk
Regeneration is not possible unless the cut ends are brought closer by surgery